1. Field of Invention
The present invention relates to an improved downhole pumping system utilizing an electric motor. More particularly, the invention concerns a system for extracting fluids from a well by using an induction motor coupled to a variable speed pulsewidth modulated (PWM) inverter via a non-gap transformer.
2. Description of Related Art
Induction motors are widely used today for a variety of different functions, including a substantial number of industrial purposes. In fact, induction motors are used nearly exclusively in tasks requiring electric motors, except in low horsepower applications. For example, induction motors have been used with considerable success in downhole drilling applications, such as deep well pumping operations. When used in the oil field, induction motors provide many advantages, such as their low cost, low power requirements, and low maintenance needs.
In some applications, induction motors receive electrical power in the form of a "line voltage" received directly from a power line of an electrical power company. Although this arrangement is beneficial in many cases, it has several drawbacks. For instance, when an induction motor is directly connected to the power line, the motor operates at one speed, in response to the frequency and amplitude of the line voltage. In downhole pumping applications, this will result in the motor pumping oil at a single rate. If multiple speed pumping is desired, this arrangement may be inadequate.
High levels of current are another problem that might be encountered when an induction motor is directly connected to a power line. In particular, when an induction motor is started, high levels of current are often required. Therefore, one must ensure that the power lines are able to supply the required starting current. In many cases, power lines with high current capacity are more expensive, since the cost of electrical service is typically related to the maximum number of amps to be supplied.
As a result of these limitations, many have installed variable speed drives between the power line and the induction motor. Typically, the variable speed drive and a drive controller of a selected type are operatively connected between the power line and a transformer. The transformer is utilized to drive the motor, and more particularly to step up the level of voltage and reduce the current supplied to the motor. This is especially important in applications such as downhole pumping operations, where a long cable connects the transformer to the motor; in these situations, the transformer helps prevent excessive current from flowing in the long cable. The variable speed drive provides more flexibility in controlling the motor's speed. One example of such a drive is a "six-step drive," which operates by providing a square wave of variable frequency and amplitude.
Six step drives still have a number of problems, however. For example, a six step drive will often produce high harmonic losses in the motor that it drives. In addition, a six step drive is more likely to damage a motor. As can be shown by Fourier analysis, a square wave is made up of multiple sinusoids of different frequencies. Accordingly, since each electrical motor is vulnerable to electrical signals of a particular frequency, a six-step drive is more likely to produce that particular frequency, and damage the motor, particularly during starting, when the fundamental frequency is low. This effect is especially important in downhole pumping applications, since long, thin, downhole pumping motors are more likely than other configurations to exhibit torsional resonance.
In contrast to six-step drives, another approach is the pulsewidth modulated (PWM) drive. Like a six-step drive, a PWM drive is operatively connected between a power line and a transformer that drives a motor. However, unlike a six-step drive, a PWM drive generates a rectangular voltage signal having a variable on-time (FIG. 1), to simulate an equivalent sinusoidal signal (FIG. 2); the equivalent sinusoidal signal may represent the electrical driving frequency (f.sub..omega.) of the motor. The frequency of the PWM voltage signal (f.sub.PWM), called the "chopping frequency," is typically constant.
One approach that is used to develop rectangular voltage signals for PWM drives is the "sine-triangle" scheme. As shown in FIG. 3, this method designates high and low periods of a rectangular voltage signal 301 based upon the intersection between a triangular wave 302 having the desired chopping frequency (f.sub.PWM), and a sinusoidal signal 304 having the desired electrical driving frequency of the motor (f.sub..omega.). The rectangular signal 300 is (1) high when the sinusoidal signal 304 is greater than the triangular wave 302, and (2) low when the sinusoidal signal 304 is less than the triangular wave 302.
With PWM drives, then, a scheme such as the sine-triangle scheme is used to determine the pattern with which the PWM drive will apply power to the motor. To further define how power is applied to the motor, some systems use "vector control" technology. Vector control technology facilitates direct control over the motor's flux and torque. In particular, vector control technology represents flux and torque as vector quantities having perpendicular "Q" and "D" components. Therefore, torque is expressed as shown in Equations 1 and 2 (below). The ".alpha." symbol is used to designate "proportional to." EQU torque .alpha. flux.sub.D .multidot.current.sub.Q -flux.sub.Q .multidot.current.sub.D 1! EQU torque .alpha. .PHI..sub.D .multidot.I.sub.Q -.PHI..sub.Q .multidot.I.sub.D 2!
I.sub.D is called "flux producing current" and I.sub.Q is called "torque producing current." By utilizing a rotating reference frame, .PHI..sub.Q may be maintained at zero, reducing Equation 2 to Equation 3 (below). EQU torque .alpha. .PHI..sub.D .multidot.I.sub.Q 3!
Thus, one benefit of vector control technology is that it facilitates independent control of flux producing current and torque producing current. Another benefit of vector control technology is its improved damping of mechanical resonances in the motor. Vector control theory is explained more completely in Blaschke's treatise, entitled "Das Prinzip der Feldorientierung, die Grundlage fur transvector-Regulung von Drehfeldmachinen," Siemens Zeitschrift, Vol. 45 (1970), pp. 757-760.
Although PWM drives provide a number of benefits, such as avoiding the potentially damaging harmonic frequencies generated by six step drives, conventional PWM drives may present certain problems in some applications. One problem is that PWM drives generate direct current (D.C.) offsets due to slight switching time biases and a beat-like phenomenon between the fundamental frequency and the chopping frequency. These small offsets will saturate a non-gapped transformer. In particular, if the total on-time of the positive rectangular voltage signals 400 (FIG. 4) is not equal to the on-time of the negative rectangular voltage signals 402, the sinusoidal equivalent signal 404 will be uneven, and a current signal 500 (FIG. 5) having a D.C. offset 502 will be created.
This condition may easily occur when the sine-triangle scheme is used. Specifically, since the triangular wave 302 and the sinusoidal signal 304 may be asynchronous, the positive and negative on-times of the rectangular signal 300 are not necessarily equal. As a result, the sinusoidal equivalent of the rectangular wave 300 may be non-symmetrical, resulting in a D.C. offset current. Therefore, although the sine-triangle approach may be adequate for driving a motor with a gapped transformer, or for directly driving the motor, this approach is limited when used to drive a motor via a non-gap transformer. Thus, in applications where a transformer must be used with a PWM drive, such as in downhole applications, the transformer must be a gapped transformer, even though non-gapped transformers are much less expensive.
One problem with variable speed drives, both six-step and PWM, is that they often have difficulty in starting highly loaded motors. This predicament is especially likely to arise in downhole pumping applications, where motors sometimes become stuck, and consequently highly loaded. Variable speed drives typically use a "constant-volts-per-Hz" relationship between applied frequency and voltage. While this scheme may drive the motor properly at high speeds, it does not perform well at startup; in some cases, a motor may not start, resulting in thermal damage to the motor, due to prolonged high current without self-pumping cooling.